The Shiga toxin (Stx)-producing Escherichia coli (STEC) serotype 0157:H7 is the most common infectious cause of bloody diarrhea, or hemorrhagic colitis (HC), in the U.S. with an estimated incidence of 63,153 cases per annum. Moreover, the hemolytic uremic syndrome (HUS), a sequela of STEC infection, is the most frequent basis for acute kidney failure in U.S. children. In addition, 0157;H7 infection is the leading cause of death in children aged 0-4 years from bacterial infections acquired through food. STEC may produce Stx1 and/or Stx2 and/or a variant of Stx2 (Stx2c and Stx2dact are the most important subtypes associated with human disease). Due to the potential severity of STEC infection, the very low 50% infectious dose, and the easy transmissibility from person to person, 0157:H7 is considered a category B biological threat by the CDC. Eariier this year, a large outbreak with more than 4000 illnesses and 800 cases of HUS occurred in Germany; the outbreak was caused by a ?. coli O104:H4 strain that makes Stx2. The O104:H4 isolate from the outbreak is an enteroaggregative E. coli (EAEC) strain that acquired the phage that carries sfx2, and, as such, can also be considered an STEC. The long-term goals of this project are to define at the molecular, cellular, and whole animal levels the pathogenic mechanisms by which STEC cause disease and to test strategies to prevent and treat these illnesses. The specific aims are to: 1) examine the relative role(s) of Stx2 and Stx2c in 0157:H7 colonization and in toxicity for mice; 2) investigate the mechanisms of enhanced toxicity of elastase-treated (ET)-Stx2dact; 3) evaluate the role of diet in host (mouse) response to STEC; and 4) determine the transmissibility of the phage that carries stX2 in O104:H4 with or without ciprofloxacin (cpx) and the impact of cpx treatment on O104:H4 disease.